A simple gist to demonstrate How to change the App Language in Android.

MyContextWrapper.java

public class MyContextWrapper extends ContextWrapper {

    public MyContextWrapper(Context base) {
        super(base);
    }

    @SuppressWarnings("deprecation")
    public static ContextWrapper wrap(Context context, String language) {
        Configuration config = context.getResources().getConfiguration();
        Locale sysLocale = null;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            sysLocale = getSystemLocale(config);
        } else {
            sysLocale = getSystemLocaleLegacy(config);
        }
        if (!language.equals("") && !sysLocale.getLanguage().equals(language)) {
            Locale locale = new Locale(language);
            Locale.setDefault(locale);
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                setSystemLocale(config, locale);
            } else {
                setSystemLocaleLegacy(config, locale);
            }

            context.getResources().updateConfiguration(config, context.getResources().getDisplayMetrics());
        }
        return new MyContextWrapper(context);
    }

    @SuppressWarnings("deprecation")
    public static Locale getSystemLocaleLegacy(Configuration config) {
        return config.locale;
    }

    @TargetApi(Build.VERSION_CODES.N)
    public static Locale getSystemLocale(Configuration config) {
        return config.getLocales().get(0);
    }

    @SuppressWarnings("deprecation")
    public static void setSystemLocaleLegacy(Configuration config, Locale locale) {
        config.locale = locale;
    }

    @TargetApi(Build.VERSION_CODES.N)
    public static void setSystemLocale(Configuration config, Locale locale) {
        config.setLocale(locale);
    }
}

usage in Activity.txt

@Override
    protected void attachBaseContext(Context newBase) {
    //fetch from shared preference also save to the same when applying. default is English
        String language = MyPreferenceUtil.getInstance().getString(MyConstants.PARAM_LANGUAGE, "en");
        super.attachBaseContext(MyContextWrapper.wrap(newBase, language));
    }
    
    Call this activity Lifecycle method in the Activity.